If the wheel is flush with the wheel arch (not hellaflush, but not sticking out beyond the arch), how are rocks shooting 2" wide of the body and turning in mid air to come back around into the car?

I'm only seeing mud on the rocker and lower quarter panel where the body work curves inward from the edge of the wheel. A mud flap coming down vertically to cover this gap makes sense, but I am failing to see the physics behind the necessity of coming 2" out past the extent of the top of the wheel arch.